Japanese Patent Application Laid-Open Publication No. 2012-027198 (Patent Document 1) describes an optical semiconductor device including a semiconductor layer made of intrinsic semiconductor, an optical waveguide which is a part of the semiconductor layer, a first impurity region of a first conductivity type formed in the semiconductor layer on one side of the optical waveguide, a second impurity region of a second conductivity type formed in the semiconductor layer on the other side of the optical waveguide, and a capacitor having a part of the second impurity region as a lower electrode.
Also, Japanese Patent Application Laid-Open Publication No. 2015-191031 (Patent Document 2) describes an optical device including an SOI substrate in which an insulating layer with a thickness of 200 nm or less is buried, an optical waveguide made of a III-V compound semiconductor material formed on the SOI substrate, and a light leakage prevention layer formed inside the SOI substrate below the optical waveguide for preventing leakage of light from inside of the optical waveguide to the SOI substrate.
Further, Japanese Patent Application Laid-Open Publication No. 2015-526883 (Patent Document 3) describes a method and a structure for providing a silicon-on-insulator substrate in which a photonic device is formed and a core material of a waveguide is optically decoupled from a supporting substrate by a shallow trench isolation region.